The lactose repressor is a genetic control protein which performs its function in Escherichia coli by binding to a specific region of the DNA. In response to signal molecules (sugars), a conformational change in the protein occurs, and the protein's affinity for the specific DNA site is decreased. In this manner, the lac region of the DNA is freed for the synthesis of the corresponding mRNA. Elucidation of the molecular events which occur during this process is required to achieve an overall understanding of the induction mechanism. Correlation of the structure of the protein with the functions it performs is essential to define these molecular interactions. Fluorescence techniques are a versatile and sensitive mode to determine structural alterations in response to ligand binding (DNA, signal molecules), to assess the characteristics of local environments, to measure binding and kinetic parameters for this system, and to monitor interactions between different regions of the molecule. By combining observations of intrinsic tryptophan fluorescence in the repressor protein with measurements on fluorophores which can be chemically introduced into the protein, the essential function of genetic control mediated by the repressor can be examined. Insights gained in this fashion may allow predictions concerning other systems and potential progress toward an overview of similar functions in more complex systems.